Liquid crystalline compounds can exhibit optical anisotropy by changing the aligned state in a fixed direction by external stimulation. Liquid crystal displays used for display screens in personal computers or portable phones are prepared on the basis of this property. Known methods for aligning liquid crystalline compounds in a fixed direction include rubbing the surface of the substrate with a cloth such as nylon or rayon. However, liquid crystalline compounds are not always aligned by rubbing any substrate, but they are normally aligned by rubbing a layer called alignment film on the substrate. Films with stabilized optical anisotropy thus obtained by aligning a liquid crystalline compound in a fixed direction are attracting attention as very useful films for improving various optical properties of liquid crystal displays. For example, films having hybrid aligned discotic liquid crystals on a polymer film such as a triacetyl cellulose film can improve viewing angle characteristics of TN-type liquid crystal displays. Films having horizontally aligned nematic liquid crystals on a triacetyl cellulose film have similar properties to those of uniaxially oriented polycarbonate retardation films and can improve the contrast of STN-type liquid crystal displays. Some of such films having an aligned liquid crystalline compound layer on a polymer film have special performance that cannot be obtained with conventional retardation films prepared by uniaxially or biaxially orienting a polymer film. Such films having an aligned liquid crystalline compound layer on a polymer film can be obtained by e.g. directly rubbing the polymer film or forming a suitable alignment film and rubbing its surface, then forming a layer of a liquid crystalline compound, turning the liquid crystalline compound in a specific aligned state and then stabilizing the aligned state.
Materials commonly used for alignment films include polyimide compounds. In order to form a polyimide into an alignment film, e.g. a polyamic acid compound as a precursor thereof is dissolved in a solvent and applied on a substrate, and then imidated by heat treatment at a high temperature to form an alignment film consisting of a polyimide compound layer. This imidation is normally performed at a high temperature of 250° C. or more so that normal polymer films cannot be used as substrates in view of heat resistance. Recently, a solution to this problem has been proposed by using solvent-soluble polyimide compounds, but solvents in which polyimide compounds are soluble are limited and they may dissolve or swell the surfaces of polymer films if they are used as substrates so that the surface properties may be seriously damaged to affect the alignment of the subsequent liquid crystalline compound. Otherwise, when a retardation film comprising a polymer film having a liquid crystalline compound layer is prepared by applying a solution of a liquid crystalline compound on a rubbed alignment film of a solvent-soluble polyimide compound, the rubbing effect will be lost and the liquid crystalline compound will not be aligned if the solvent is the same as used to dissolve the polyimide compound. Thus, it would be highly desirable to provide an alignment film that can be used even on less heat- and solvent-resistant polymer films.
Polymer films contain unreacted monomers and materials such as antistatic agents, UV absorbers, leveling agents, adhesion improvers, antiblocking agents, plasticizers for conferring formability or flexibility on the films, etc. However, some kinds of these materials may migrate to the surfaces of the films with time. This phenomenon is accelerated especially in high-temperature and high-humidity atmospheres. When a retardation film having a layer of a certain type of aligned liquid crystalline compound formed on a polymer film is used, therefore, these materials may migrate to the liquid crystalline compound layer to cause change in retardation value or insufficient alignment or otherwise affect the optical properties of the aligned liquid crystalline compound. Such a problem is especially significant when a retardation film having a liquid crystalline compound layer is prepared using a triacetyl cellulose film containing a phthalic acid ester such as triphenyl phosphate (hereinafter referred to as TPP), biphenyl diphenyl phosphate or ethyl phthalyl ethyl glycolate as a plasticizer.